Comment-errors
This project is not a full language, but rather adds first-class comments to an existing language: Rust.
It is a proc-macro that hijacks doc comment syntax inside function definition to use them as templates for nicer error-reporting.
The proc-macro source is in comment-errors. comment-errors-test contains a few examples.
Running the example
cargo run -p comment-errors-testNote, on NixOS I was having issues getting the proc-macro to compile properly and ended up doing:
cargo +nightly run -Ztarget-applies-to-host -p comment-errors-test